1. Field of the Invention
The invention relates to the production of annular parts by means of a generally flat metal sheet and whose initial shape is predetermined in order to form, once deformed, the annular part to be obtained. The invention is advantageously used when machining operations are to be carried out on said part. The invention has more particularly been developed for producing a slightly conical, front ferrule of an aircraft turbojet engine casing.
2. Description of the Prior Art
Double flow turbojet engines constitute one propulsion engine type for aircraft. Their overall shape can be likened to a cylinder having a length of several meters, the external diameter varying constantly as a function of the construction of the different parts of the engine. For example, at the coldest flow pipe surrounding the engine, the casing has a shape which widens slightly and then becomes cylindrical, before finally reassuming a conical shape in order to reduce the diameter of said coldest flow pipe. Therefore the engine casing is often constituted by a succession of annular ferrules, whose shapes have varying degrees of complication and on which numerous machining operations take place with a view to fixing the different accessories (inspection means, ducts, control means, etc.). Moreover, these machining operations frequently include the hollowing out of the ferrule in order to reduce its weight.
For example, a front ferrule of a casing at the coldest flow pipe can be constituted by a slightly conical, titanium ferrule, whose diameter can be between 500 and 1000 mm. It is often obtained from a metal sheet with a thickness close to 7 mm and which is then hollowed out by a machining operation in order to lighten the same and provide the fixing of various accessories, while still leaving ribs in order to rigidify the part. Such a part can be obtained in different ways.
One conventional method for shaping the metal sheet, which is known as "ferruling", consists of cambering or bending the sheet between several rollers positioned on either side thereof. The definitive radius of curvature is obtained by the successive passage of the sheet between these rollers. Once bent, the ferrule is terminated by welding in accordance with two generatrixes using conventional equipment. This method makes it necessary to carry out machining operations following the cambering or bending, because the pressure forces of the rollers would lead to the flattening or inclination of numerous ribs resulting from the machining. It is then necessary to machine the part following said bending operation, which is relatively difficult and expensive. Thus, the machining of a ferrule with a diameter of approximately 800 mm and having at numerous locations a thickness of 1 or 2 mm is very difficult. However, such a machining can be brought about chemically, the ferrule being immersed in a large tank provided for this purpose, but said method is very expensive and difficult to perform.
Another method used consists of carrying out bending by successive folds using a conventional folding machine. The folds are made along the generatrixes, which have a reduced inertia compared with the others. This method suffers from the disadvantage that folding marks on the ribs are left behind after said bending operation. Moreover, in this method, the metal is exposed to stresses exceeding the yield strength or modulus of elasticity of the metal.
The aim of the invention is to produce such parts by shaping the metal sheet into a ferrule after carrying out all necessary machining operations on the flat sheet, so as to avoid using chemical working or machining.